There is known a NOx occlusion reduction catalyst device, which absorbs oxides of nitrogen (NOx) included in an exhaust gas of an internal combustion engine and purifies the exhaust gas. The NOx occlusion reduction catalyst device absorbs NOx included in the exhaust gas when the internal combustion engine is operating in a zone in which the air/fuel ratio is lean (during lean combustion). However, the NOx occlusion reduction catalyst device is no longer feasible for absorbing NOx in the exhaust gas when the amount of NOx absorbed reaches a saturated state. In this case, NOx will be emitted into the atmosphere together with the exhaust gas. Thus, a process of regenerating the NOx occlusion reduction catalyst is carried out by temporarily operating the internal combustion engine under fuel excessive conditions (rich combustion), whereby NOx absorbed in the NOx occlusion reduction catalyst device is reduced to nitrogen gas N2.
As to regeneration of NOx occlusion reduction catalyst, Patent Document 1 discloses an art of changing the air/fuel ratio to a rich mixture by reducing the degree of opening of a throttle valve to reduce the amount of intake air introduced into the engine when a rich combustion operation is performed.
In valve driving systems of the internal combustion engines, there is known a variable valve driving mechanism, which changes the period of time during which a cam acts on a valve to change the working angle of the valve. In Patent Document 2, there is disclosed a variable valve driving system of an internal combustion engine in which a control shaft interposed between a cam and a valve is rotated by a drive source, whereby the magnitude of valve lift with respect to the rotational position of the cam is changed and the working angle of the valve is thus changed. In Patent Document 3, there is another variable valve driving system in which an annular disk is interposed between a flange part of a sleeve joined to a drive shaft and a flange part of a camshaft and the rotational center of the annular disk is eccentric to generate unequal velocity rotation to change the working angle of the valve.